What was found
BNA - FLT 402 - A/C WAS AT CRUISE, EN ROUTE TO ALT FROM BNA, APPROX 35 MINUTES INTO THE FLIGHT BOTH HYDRAULIC PRESSURE GAUGES STARTED TO FLUCTUATE AND THEN WENT OT ZERO PRESSURE. THE A/C INCURRED A HYD SYSTEM FAILURE. THE A/C RETURNED TO BNA. AN EMERGENCY LANDING WAS DECLARED AN THE LANDING GEAR AND FLAPS WERE ACTIVATED BY MEANS OF THE EMERGENCY HYD SYSTEM. THE A/C LANDED AND RETURNED SAFELY TO THE GATE. THE RIGHT HYD PUMP RETURN LINE HAD RUPTURED AS A RESULT OF CHAFING THE COMPRESSOR DRAIN LINE. THE LINE WAS REPLACED AND ALL ALSO NR 1 AND NR 2 ENGINE DRIVEN HYD PUMPS. THE SYSTEM FILTERS WERE INSPECTED AND REPLACED. NO EVIDENCE OF METAL NOTED. THE ENTIRE A/C HYD SYSTEM WAS BLED IAW THE A/C MM. GEAR, F